The Michigan and Lima-Indiana districts are closely related from a structural and production standpoint. The folds are comparatively gentle, asymmetrical, en échelon, and superimposed upon the major structural features of the bifurcating limbs of the Cincinnati arch and the isolated Michigan synclinal basin. The production is obtained largely from porous dolomitic limestone reservoirs occurring at or beneath surfaces of disconformity. In Michigan the oil comes principally from Devonian rocks, whereas in the Lima-Indiana district the reservoir beds are of Ordovician age.
The Michigan “basin” was originally a geosyncline probably having its inception during Keweenawan time when the great Keweenawan disturbance occurred. Subsequent folding took place parallel with the long axis of the downwarp which parallels the direction of the Kankakee arch and the Wisconsin positive element. Tilting of the region and the uplift of the Cincinnati arch by tangential pressures from other directions brought about periodic isolation of the basin, gave rise to evaporite conditions, and created its present shape.
Individual structures in the region do not seem to be accentuated at depth, and accumulation is intimately associated with faulting, cross folding, and unconformable overlap. The intraformational surfaces of disconformity have modified deformation by their irregularities, aided accumulation by bringing source and reservoir rocks into juxtaposition, and furnished porous beds with solution porosity to serve as reservoirs.